Worn brake lining alarm circuit

ABSTRACT

A negative resistance diode and an alarm device are connected in a series circuit with a battery whose terminal potential is higher than the threshold switching voltage of the diode. A worn brake lining detector circuit is connected in shunt with the series circuit. Normally, the shunt circuit draws current from the battery so that the voltage applied across the diode is below its switching voltage but above its holding voltage, and the alarm is not actuated. If even a momentary interruption of the shunt circuit occurs, the full battery potential is applied to the diode, thereby switching the diode into a conducting state to actuate the alarm. Even if the shunt circuit is subsequently closed by electrical contact with the rotating brake disc, the holding current through the diode is maintained, so that the diode remains conducting and the alarm remains actuated.

[451 Mar. 4, 1975 WORN BRAKE LINING ALARM CIRCUIT [75] Inventor:

[73] Assignee:

Yasuo Kita, ltami, Japan Sumitomo Electric Industries Ltd., Osaka, Japan [22] Filed: July 20, 1973 [21] Appl. No.: 381,196

[30] Foreign Application Priority Data Prinmry E.\'aminerAlvin H. Waring Arto'rney, Agent, or Firm sughrue, Rothwell. Mion, Zinn & Macpeak ABSTRACT A negative resistance diode and an alarm device are connected in a series circuit with a battery whose terminal potential is higher than the threshold switching voltage of the diode. A worn brake lining detector circuit is connected in shunt with the series circuit. Normally, the shunt circuit draws current from the battery so that the voltage applied across the diode is below its switching voltage but above its holding voltage, and the alarm is not actuated. If even a momentary interruption of the shunt circuit occurs, the full battery potential is applied to the diode, thereby switching the diode into a conducting state to actuate the alarm. Even if the shunt circuit is subsequently closed by electrical contact with the rotating brake disc, the holding current through the diode is maintained, so that the diode remains conducting and the alarm remains actuated.

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PATENTEDW 1915 FIG. I

7 Q \mT m VOLTAG E WORN BRAKE LINING ALARM CIRCUIT BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to the field of electrical alarm circuits for detecting a worn condition of friction brake linings, and, more particularly, to such a circuit which is fail-safe in operation.

2. Description of the Prior Art A prior art worn brake lining alarm circuit of the type in which a warning is issued when an electric detecting circuit .is interrupted when the lining wears down beyond a predetermined working limit is disclosed in Japanese Publication No. 8,499/1972, filed Feb. 13, 1968. Such a prior art circuit is superior to other prior art circuits in that any interruption in any portion of the detection circuit, including the lead wire to an alarm device is indicated as an alarm condition. Such fail-safe operation is important since the detection circuit may be interrupted by causes, such as vibration, etc., other than a wearing down of a brake lining beyond its working limit.

However, in the past a considerable amount of hardware was required to achieve this fail-safe function with a loop working limit detection circuit of the type disclosed in German Patent Publication Serial No. 1,045,259, filed July 12, 1957 when, after an initial interruption of the detection circuit, the detection circuit is subsequently closed by the linings wearing down to its working limit so that the loop circuit is shortcircuited to the grounded rotating disc of the brake assembly.

SUMMARY OF THE INVENTION The object of this invention is to provide an improved worn brake lining alarm circuit, employing the principle of the interruption of a conductor loop detector circuit for detecting that a lining has worn down beyond its working limit, and which issues a continuous warning once the detector circuit is even only momentarily interrupted, even when subsequent wearing of the lining to its working limit causes a loop to electrically contact the grounded brake assembly to reclose the detector circuit.

Another object is to provide such an improved circuit including a negative resistance diode to accomplish the result stated in the foregoing object.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-3 are schematic circuit diagrams of preferred embodiments of the invention.

FIG. 4 is a current and voltage characteristic curve diagram of a negative resistance diode employed in this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates one preferred embodiment of the present invention for actuating an alarm when the working limit of a friction lining of a disc brake assembly has been exceeded. In FIG. 1, a disc brake assembly includes a rotating brake disc 12 and a pair of brake pads 14 and 16 having mounted thereon friction linings l8 and 20 respectively. An insulated electrical conductor 22 has formed therein two fixed loops 24 and 26. Loop 24 projects through the pad 14 and into the back of the lining 18 to a point corresponding to the working limit of the lining 18. That is, the point at which the loop 24 is embedded in the back of the lining 18 is the point beyond which the brake lining should not wear, and it is at this point that it is desired to actuate an alarm to indicate that the lining has worn down to its working limit. In like manner, loop 26 is embedded in the lining 20 at a point corresponding to the working limit of that lining.

The circuit formed by conductor 22 and loops 24,26 functions as a detector circuit for detecting that at least one of the brake linings has worn down to its working limit. As described in West German Publication Ser. No. 1,045,259, for example, each loop is adapted to be abraded by the rotating brake disc 12 when the working limit is reached, thereby causing an interruption of the detector circuit.

In accordance with this invention, a resistance 28, an electrically operated warning device 30, such as an electric lamp, a luminous diode, a buzzer, etc., and a negative resistance diode 32 are-series-connected with a power source 34, such as a battery, which generates a terminal voltage equal to or greater than the threshold switching voltage Vth of the negative resistance diode 32.

A resistance 36, conductor 22, and loops 24, 26 are series-connected and are shunt-connected with respect to the series circuit formed by the alarm device 30 and the negative resistance diode 32.

In the operation of the circuit shown in FIG. 1, the voltage of the source 34 is divided by the resistances 28 and 36 so as to apply to diode 32 a voltage lower than its switching voltage Vth, but higher than its holding voltage Vh. The diode 32 is a known negative resistance diode having current-voltage characteristics as shown in FIG. 4. Consequently, under normal conditions, the diode is non-conducting so that the warning device 30 is not activated.

However, if any interruption occurs in any part of the detector circuit connected in shunt with the seriesconnected alarm 30 and diode 32, the full, undivided source voltage is applied to the diode 32 so that a voltage higher than the switching voltage Vth is applied thereto, with the result that the diode conducts and the alarm device 30 is activated. Even if the shunt-circuit which has been interrupted by the breaking of a loop through abrasion is reclosed by electrical contact of the loop with the grounded disc upon subsequent application of the brake, a current larger than the holding current Ih of the diode continues to flow through the alarm device 30 to maintain it activated. This result occurs since the divided voltage is always greater than the diode holding voltage Vh, even though it is lower than the switching voltage Vth. That is, the momentary interruption of the shunt circuit is memorized by diode 32. Furthermore, the alarm is fail-safe, since it is activated in the above manner even when the conductor 22, which is a part of the shunt-circuit, is broken by any cause.

FIG. 2 is a further preferred embodiment wherein the internal resistance of the alarm device 30 is used, instead of the resistance 28, to form the voltage divider. In this case, a resistance 38 is required, and its value is selected so that the divided voltage applied to diode 32 is greater than Vh, but less than Vth. In this embodiment, the loop detector circuit is connected directly in shunt with the diode 32.

FIG. 3 is a still further simplified preferred embodiment wherein a recently developed negative resistance luminous diode 40 which operates as a luminous of light-emitting diode having negative resistance is used to provide both the switching function of diode 32 and the alarm function of device 30, as shown in FIG. 1.

As described in detail above, the present invention provides a working limit warning apparatus for brake friction lining, which apparatus is highly improved in safety and wherein the suspension of a warning is prevented when an interruption-type detector circuit is used.

Even though the above embodiments include an embedded loop type detector, the invention covers a failsafe alarm circuit for use with any detector circuit of the type in which an electrical interruption within the detector circuit indicates a worn brake lining; eg the type in which the grounded back plate is disconnected from the conductor 22 when the lining wears beyond its working limit.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

1 claim:

1. In combination with a worn brake-lining detector circuit of the type which is electrically interrupted when the lining wears down to a predetermined working limit, an improved alarm circuit for indicating the interruption comprising:

a. a DC power source,

b. voltage divider means connected to said power source,

c. a normally non-conducting two-terminal negative resistance diode coupled to said voltage divider means, r

d. means connecting said detector circuit in shunt with said diode; whereby when said shuntconnected detector circuit is normally uninterrupted, the divided voltage appliedto said diode by said voltage divider means is less than the threshold switching voltage, but greater than the holding voltage, of said diode; and

c. said DC. power source having a terminal potential greater than or equal to the threshold switching voltage of said diode so that, when said detector circuit is interrupted even only momentarily. said terminal voltage is applied to said diode to render said diode conducting as an indication of the interruption, said diode remaining continuously conducting even if said detector circuit is subsequently closed.

2. The improved alarm circuit as defined in claim 1, further comprising a current-actuated alarm means connected in series with said diode, and wherein said detector circuit is connected in shunt with the series combination of said diode and said alarm means.

3. The improved alarm circuit as defined in claim 1, wherein said diode is a light-emitting diode which emits light when said diode becomes conducting.

4. The improved alarm circuit as defined in claim 1, wherein said voltage divider comprises a resistor and the internal resistance of an electrical alarm means connected in series between said detector circuit and one terminal of said power source; and further comprising means connecting said diode between the other terminal of said power source and the function of said resistor and said alarm means, whereby said alarm means is current-activated only when said diode is conducting. 

1. In combination with a worn brake-lining detector circuit of the type which is electrically interrupted when the lining wears down to a predetermined working limit, an improved alarm circuit for indicating the interruption comprising: a. a D.C. power source, b. voltage divider means connected to said power source, c. a normally non-conducting two-terminal negative resistance diode coupled to said voltage divider means, d. means connecting said detector circuit in shunt with said diode; whereby when said shunt-connected detector circuit is normally uninterrupted, the divided voltage applied to said diode by said voltage divider means is less than the threshold switching voltage, but greater than the holding voltage, of said diode; and e. said D.C. power source having a terminal potential greater than or equal to the threshold switching voltage of said diode so that, when said detector circuit is interrupted even only momentarily, said terminal voltage is applied to said diode to render said diode conducting as an indication of the interruption, said diode remaining continuously conducting even if said detector circuit is subsequently closed.
 2. The improved alarm circuit as defined in claim 1, further comprising a current-actuated alarm means connected in series with said diode, and wherein said detector circuit is connected in shunt with the series combination of said diode and said alarm means.
 3. The improved alarm circuit as defined in claim 1, wherein said diode is a light-emitting diode which emits light when said diode becomes conducting.
 4. The improved alarm circuit as defined in claim 1, wherein said voltage divider comprises a resistor and the internal resistance of an electrical alarm means connected in series between said detector circuit and one terminal of said power source; and further comprising means connecting said diode between the other terminal of said power source and the function of said resistor and said alarm means, whereby said alarm means is current-activated only when said diode is conducting. 